1. Field of the Invention
This invention concerns a purification process, particularly one for separating excess of reactants from aminophosphonates.
2. Background Information
Aminoalkylene phosphonates are used as sequestering agents in for example detergents and may be made by reacting amino compounds, an aldehyde or ketone and phosphorous acid in the presence of hydrochloric acid. The reaction product is an aqueous solution of amino alkylene phosphonic acids with varying degrees of phosphonoalkylation, and excess of carbonyl compound, phosphorous acid and especially hydrogen chloride. As described in U.S. Pat. No. 3288846 and J. Org. Chem. 1966, volume 31, pages 1603-1607, the reaction product in some cases may be worked up by cooling to crystallize out the phosphonate compounds as the free acid. The products are also sold as solutions of their salts e.g. as the sodium salt and in this case the reaction product solution is treated with alkali. But the solution of free acid and salts thereof contain chloride ions which can give rise to corrosion and also in the case of the sodium salt means that the product contains sodium chloride which tends to crystallize out from concentrated salt solution. The chloride content can be reduced by distillation of hydrogen chloride from the free acid but this operation is expensive and may tend to degrade the product, and is corrosive.